Lillooet River Floodplain Mapping

Pemberton Valley Dyking District

Pemberton, British Columbia

River Engineering, Floodplain and Sediment Management

In 2010, a large landslide occurred on Mount Meager, about 65 km upstream of Pemberton Village. The slide temporarily blocked the Lillooet River and introduced a substantial amount of sand and gravel into the system. Pemberton Valley Dyking District commissioned NHC to update the floodplain mapping for the Lillooet River and tributaries, and to assess channel morphology and sedimentation processes in the context of the slide event to evaluate future potential impacts on channel aggradation and flood conveyance capacity. The geomorphic assessment provides a rationale for intensifying sediment removals. NHC surveyed 86 km of river channel and developed a two-dimensional model of the Lillooet River and tributaries to show variations in water level and velocity across the floodplain. Flood inundation, depth, and hazard maps were produced to inform administrative authorities, residents, and other stakeholders. Dike breach scenarios were modelled and critical flow thresholds for dike overtopping were estimated for emergency response planning.